A total knee prosthesis typically comprises a tibial baseplate and a femoral component with an intervening meniscal bearing component which may have medial and lateral sides. Typically the tibial plate and femoral compartment are made from a suitable metal or metal alloy, such as an alloy of cobalt and chromium, whereas the meniscal bearing component is made from a synthetic plastic material, for example ultra high molecular weight polyethylene. In some designs the meniscal bearing component is fixed to the tibial plate. However, in others it is free to float to some extent with respect to the tibial plate in an attempt to mimic more closely the natural movement of the knee. In some arrangements the meniscal component may be allowed rotary and/or sliding motion on the tibial baseplate. This movement may reduce the occurrences of dislocation of the components of the prosthesis in normal walking.
Examples of prior art prostheses which include some floating bearing surfaces include U.S. Pat. No. 6,972,039, U.S. Pat. No. 2005/0209702, U.S. Pat. No. 6,428,577, U.S. Pat. No. 6,413,279, U.S. Pat. No. 6,319,283, U.S. Pat. No. 6,296,666 and U.S. Pat. No. 6,238,434. Although these and other prior art prostheses offer advantages over earlier arrangements in providing certain degrees of freedom that are not present where the bearing surface is fixed to the tibial plate or is integral therewith, problems with dislocation can still occur, particularly if the ligaments salvaged and tensioned by the surgeon during the implantation of the prosthesis do not provide the required support. Further, where the meniscal component is allowed to have anterior-posterior translation motion, spinout of the bearing may occur.
With a view to increasing stability and to reducing the risks of spinout, prostheses have been suggested which include a post extending superiorly from the tibial component and which in use will be located in the inter-condylar space of the femur. However, these prostheses suffer from various disadvantages. In particular, the interaction between the surface of the post and the surface of the femoral component of the prosthesis can lead to metal wear which may lead to metal particles being released which in turn can lead to accelerated wear of any polymeric components possibly requiring early revision operations.
It is therefore desirable to provide a mobile bearing tibial implant comprising a tibial component and a meniscal component which when used together provide the desired range of motion in flexion, tension and in rotation and which overcome the problem associated with the risk of spinout noted in the prior art. It is further desirable to provide an arrangement which has non metal on metal components which would not lead to metal on metal wear.